Wave-signal communication system



Nov. 6, 1951 A. v. LOUGHREN 2,574,271

WAVE-SIGNAL comaUNIcA'rIoN SYSTEM Filed Avril 2, 1949 5 Sheets-Sheet l IN V EN TOR. THUR V. LOUGHREN ATTORNEY Nov. 6, 1951 A. v. LouGHRl-:N 2,574,271

WAVE-SIGNAL comuNrcATIoN SYSTEM Filed DIil 2, 1949 5 ShSStS-Sheet 2 ARTHUR V. LOUGH REN ATTORNEY NOV. 6, 1951 A, V, LQUGHREN 2,574,271

WAVE-SIGNAL COMMUNICATION SYSTEM Filed Avril 2, 1949 5 Sheets-Sheet 5 INVENTOR. ARTHUR V. LOUGHREN www ATTORNEY Nov. 6, 1951 A. v. LouGHREN WAVE-SIGNAL COMMUNICATION SYSTEM 5 Sheets-Sheet 4 Filed Avril 2, 19'49 m .mi

INVENTOR. ARTHUR V. LOUGHREN ATTORNEY Nov. 6, 1951 A. v. LOUGHREN 2,574,271

WAVE-SIGNAL comumcmlon SYSTEM Filed April 2, 1949 5 Sheets-Sheet 5 @Etzwzw a hamm j INVENTOR. ARTHUR v. LOUGHREN M AT To R NEY Patented Nov. 6, 1951 WAVE-SIGNAL COMUNICATION SYSTEM Arthur V. Loughren, Great Neck, N. Y., assignor to Hazeltine Research, Inc., Chicago, Ill., a corporation of Illinois Application April 2. 1919, serai No. 85,101

6 Claims.

This invention relates to a wave-signal communication system and, more particularly, to a system for communication between two spaced wave-signal stations at least one of which is carriedby a mobile object capable of entering various onesof different trame zones in space.

To relieve tramo congestion where large numbers of mobile objects are involved, particularly when the mobile objects are aircraft, it has been proposed to establish automatic position-indicating systems permitting closer approaches of the mobile objects to each other without danger of collision. This may be done by dividing the space available to the mobile objects into zones,

which may be fixed in space or may represent a safety zone ,Xed with reference to each mobile object to aid in the guiding thereof without collision hazards. For guiding aircraft it is convenient to establish altitude zones, so that the positions of near-by aircraft in the same altitudezone may be indicated to the exclusion of allother aircraft.

One such system using zonal trame divisions provides a transmitter for-sending wave signals coded to identify a particular tralc zone and also provides', spaced from the transmitter, a receiver for decoding these signals and utilizing them to indicate position or to send a reply signal whenever the receiver is adjusted to decode ,signals identifying that particular traffic zone. The transmitter or the receiver is aboard a mobile object, or both are carried by mobile objects, which may enter numerous tralc zones.

The code may identify the traiiic zones by dis.

tances from a xed reference point; for example, it may identify altitude zones by altitude in feet above ground or above mean sea level or by pressure altitude" in millibars of atmospheric pressure referred to the standard sea level atmospheric pressure of 1013 millibars. The transmitted wave signal is coded by modulating it with signal energy of pulse wave form having two relatively variable wave form portions. Thus, when the transmitter is aboard an aircraft, the modulation envelope of the transmitted wave may comprise two short pulses separated by an adjustable period of time automatically made proportional to the pressure in millibars indicated by a barometer on the aircraft. The receiver aboard another aircraft in the same altitude zone demodulates this wave and applies it to a decoder automatically adjusted by another barometer on that aircraft to translate the demodulated signal only when it comprises two v pulses separated by the period of time corresponding to that altitude. A signal translated by the decoder may cause transmission of a reply signal. Such a system is described and claimed in an application of Knox McIlwain, Serial No. 617,020, tiled September i8, 1945,v now abandoned.

It is an object of the present invention to provide a novel signal-translating arrangement which exhibits improved operating characteristics not possessed by the described prior arrangements.

It is also an object of the invention to provide a new and improved decoding receiver, for use in a signal-translating system involving the coding and decoding of signals representing values in an extended total range o1` values, having unusually high reliability of operation of the decoding circuits of the receiver.

It is another object of the present invention to provide a new and improved system for communication between two spaced wave-signal stations, at least one of which is carried by a mobile object, which system is capable of utilizing selectively signals designating a large range of locations or traffic zones available to mobilev objects and has improved arrangements for selecting a desired trame zone.

It is a still further object of the invention-tc provide a wave-signal receiver suitable for use in a system for communication between two spaced wave-signal stations, at least one of which is carried by a mobile object, and onel which is capable of utilizing selectively and with high accuracy of selection yzone indicative signals which designate one traflic zone within an extensive total range of such lzones available to mobile objects. f Y

Y In accordance with one form of the invention, a system for communication between two spaced wave-signal stations at least one of -which is carried lby a mobile object comprises means at one of the stations for transmitting wave-signal energy having a rstmodulation characteristic including two pulse wave-form portions with a variable time separation for designating by the values of the time separations thereof individual onesof a plurality of traffic sectors available to mobile objects and having a second modulation characteristic including one of the aforesaid two pulse wave-form portions and including another pulse wave-form portion with a variable time separation designating by the values of the time separations thereof individual ones of a plurality of traiiic zones located primarily within each of the traic sectors individually designated by the first characteristic. The system includes tion for utilizing the derived signal energy only when the time separations of the pulse waveform portions thereof correspond to a particular tralc sector and zone selectable at the other station. 4

In accordance with another form of the invention, a wave-signal receiver, for use in a system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object, comprises means for intercepting wave-signal energy having a first modulation characteristic including two pulse wave-form portions with a variable time separation for designating by the values of the time separations individual ones of a plurality of trafiic sectors available to mobile objects, said wave-signal energy also having a second modulation characteristic including one of -the aforesaid two pulse wave-form portions and including another pulse wave-form portion with a variable time separation designating by the values of the separations individual ones of a plurality of trafc zones located primarily within each of the traffic sectors individually designated by the aforesaid rst characteristic. The wave-signal re- .ceiver also includes means for deriving from the intercepted wave-signal energy including three pulse wave-form portions having the aforesaid two Itime separations. The receiver further includes means for utilizing the derived signal energy only when the time separations of the pulse wave-form portions thereof correspond to a particular traiic sector and zone selectable at the receiver.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings, Fig. 1 represents schematically a transmitter for use in a communication system embodying the present invention and Fig. 2 represents schematically a receiver-.transmitter arrangement also for use in this system; Fig. 3 graphically depicts certain related signal 'characteristics which may be obtained using the system of Figs. 1 and 2 and is used as an aid in explaining the system operation; Fig. 4 is a group of graphs having a common time scale and depicting the signals obtained in accordance with the graph of Fig. 3 under one set of conditions; Fig. 5 represents in somewhat greater detail a aumen specific transmitter of the general type shown in Fig. 1; Fig. 6 represents in somewhat greater detail a specific receiver of the type shown generally in Fig. 2; Fig. 7 is a graph corresponding to a portion of the graph of Fig. 3 and depicting in greater detail the operation of the arrangement of Figs. 5 and 6; Fig. 8 is a group of graphs having a common time scale and depicting the signals obtained in accordance with the graph of Fig. 7 under a particular set of operating conditions; and Fig. 9 is a representative display provided by Fig. 1 apparatus.

Referring now more particularly to Fig. 1 of the drawings, there is represented schematically a transmitter suitable for use at one of the stations in a system for communication between two spaced wave-signal stations ,at least one of for aircraft navigation and control.

4 which is carried by a mobile object. While the present invention has utility in a wide range of applications, it has particular utility in a system in which one or both of the wave-signal stations is or are carried `aboard aircraft and will therefore be described in connection with a particular form of system especially desirable In such a system, one of the two spaced stations is an interrogating station for transmitting interrogating signals to the other station, which may be termed a replier or a transpondor station. The transpondor station isA adopted to receive the interrogating signals and to transmit reply signals in response thereto. The reply signals are received by a responser arrangement at the interrogating station, the responser utilizing the reply signals to make available to the operator of the interrogator-responser station information obtained from the reply signals concerning the transpondor station.

In one form of the system last mentioned, the interrogating signals serve merely to elicit reply signals from all transpondors within range and to initiate a cycle of operation Vof the responser arrangement at the interrogator station. In this form of transpondor system the reply signals may be coded to give desired information,-

such as identity of the transpondor station. However, in the preferred form of system the interrogating signals are coded and only those transpondor stations having suitably adjusted decoder apparatus can transmit reply signals. In this way the number of reply signals may be decreased greatly. v

Accordingly, vthere is represented in Fig. 1 an interrogator-responser station constituting one of the two spaced wave-signal stations required for operation of a communication system of the transpondor type and including an arrangement for transmitting coded interrogating signals. This one station includes a pulse generator II having an output circuit coupled to the input circuit of an attenuator and wave shaper I2.

Unit I 2 has an output circuit coupled to an inputci'rcuit of an interrogator transmitter I5, the

output circuit of which is coupled to an antenna system I6. The output circuit of pulse generatorA The interrogator station also includes a barometric device 24. This device is responsive to the ambient atmospheric pressure at the interrogatingA station and is coupled mechanically to the fine pulse-timing circuit I9 and to the coarse pulse-timing circuit 22 to control the operation of these units. Unitsv I8--24 provide a. signalcoding arrangement. 'I'here also is provided a. responser arrangement, comprising a receiver 28, located at the interrogating station and adapted to receive reply wave signals. The receiver 28 is coupled to a suitable receiving antenna and has an output circuit coupled to a position-display unit 29 for utilizing reply wave signals to provide an indication of the position of the replying or transpondor station. Unit 29 has an additional input or control circuit connected to the output circuit of the coarse pulse-timing circuit 22.

Before considering the operation of the interrogator-responser station shown in Fig. l, the transponder arrangement at the other station of the system will be considered. Refer to Fig. 2, a transponder station is schematic y represented as including means for intercepting Wave-signal energy radiated by the interrogator transmitter and for deriving the modulation components of this wave-signal energy. This means includes an antenna system 30 and an interrogating-signal receiver 3|. The receiver 3| has an output circuit coupled to one of the input circuits of a mixer 32. Unit 32 inturn is coupled to a reply transmitter 35, which has an antenna 36. Mixer 32, transmitter 35 and antenna 36 form means coupled to the receiver 3| for utilizing, under certain conditions te be discussed in detail hereinbelew, the signals derived in receiver 3| from the intercepted interrogating signals.

The transponder station also includes two time-delay means. One of these means,l which is coupled to the output circuit of the receiver 3I`through an isolating'resistor 38, comprises a fine pulse-delay circuit 39 having an output circuit coupled to the input circuit of a pulse generator and amplier 40. The output circuit of unit 40 is' coupled te a second input circuit of the mixer 32. The other of the time-delay means, which also is coupled to the output circuit of receiver 3| through an isolating resistor 4I, comprises a coarse pulse-delay circuit 42 having an output circuit coupled to the input circuit of a pulse generator and amplifier 43. The output circuit of unit 43 is coupled to a third input circuit of the mixer 32. An altitude-responsive barometric device 44, which .is responsive to the ambient atmospheric pressure at the transponder station, is coupled mechanically to the fine pulsey delay circuit 39 and to the coarse pulse-delay circuit 42 tocontrol the operation of these units.

Describing generallythe operation of thecemplete communication systeml of Figs. 1 and 2, pulse generator Il of the interrogating transmitter generates a series of short pulses at a repetition frequency which may be determined in the generator II itself. The rectangular wave shape of these pulses is improved by unit I2 which employs well-known wave-shaping circuits, and the p'ulses may be attenuated somewhat if this is found desirable for a reason te be mentioned hereinbelow. The resulting attenuated and shaped pulses modulate a radio-frequency signal generated in unit I5, and the modulated signal is radiated from antenna system I6. This radiated signal is intercepted at the antenna 30 of the transponder and the pulse-modulation compenents thereof are derived in the receiver 3|. If these pulse signals are accepted by the mixer 32 in a manner more fully to be explained presently, this unit translates a trigger pulse or pulses for modulating a reply wave signal generated in the transmitter 35. This transmitter radiates the modulated signal from antenna 36, and it is received by the responser receiver 28 of the Fig. l arrangement. After demodulation in unit 28, the reply signals are applied to position-display unit 29 where they are effective in conjunction with interrogating pulses supplied directly to unit 29 from the timing circuit 22 to indicate the position or identity, or both, of the transponder station.

For a more detailed description of the operation of the transponder system of Figs. l and 2, reference is had to the graph of Fig. 3. The communication system represented in Figs. 1 and 2 is one in which the interrogator station or the transponder station. or both, is carried by a mobile object which travels into or through numerous traffic zones into which the space available to mobile objects is divided according te a prearranged plan. In one specific system, at least one of the interrogator and transponder stations is aboard an aircraft and, for purposes of aircraft navigation and control, altitude traiilc zones are established under the control of barometric devices. The response of the barometric devices' is directly proportional to altitude, and

^ all are preferably calibrated, preset and sealed at the factory so that all of the barometric devices within a reasonably large area read alike at a given altitude without regard to the specific Value of barometic pressure existing at that altitude at a given time. Traflic may then be controlled in accordance with the "pressure altitudes indicated by the barometric devices.

In dividing the air space available to aircraft into altitude divisions having upper and lower altitude limits, in conformity with the present invention, the total altitude range to be covered is first divided inte a plurality of rather large divisions referred to herein as altitude trafc sec' tors. Since these divisions or sectors are too large for adequate control of large numbers of aircraft, each sector is subdivided into a plurality of altitude traffic zones. .c

.In the following description of the system operation, it will be assumed that both the interregating station and the responser station are carried by individual aircraft since that is a type of system which permits a description of one rather flexible mode of operation involved. Referring now more particularly to the graph of Fig. 3, the times of occurrence of the various signals produced by the arrangements of Figs. l and 2 are indicated on the axis of abscissae. The

upper half of the graph has an axis of ordinates scale representing the altitude of the interrogator station of Fig. 1, the top of the graph correspending to the highest altitude covered by the system. The lowest altitude of the interrogator which is indicated on the graph corresponds to an atmospheric pressure somewhat greater than standard sea level pressure in order to provide a safety margin with regard to atmospheric pressure disturbances.

The time of occurrence of the pulses generated in the pulse generator II does not vary with any altitude control and thus may be represented as occurring at time to for all altitudesas indicated in Fig. 3 by the line 5|. It will be understood that the pulse generator II of the interregator station generates a signal of repeated pulse Wave form and, therefore, that subsequent pulses produced by the generator II may also be assigned the time to since each interrogating and response cycle of the transponder system is repeated at the repetition rate of pulse generator I. A pulse occurring at time to is applied not only directly to wave shaper 2 but; also through isolating resister I8 te the fine pulse-timing circuit I9, in

attenuation and degradation oi wave form which the timing circuit may cause. Similarly, the 'same pulse also is applied through isolating resistor 2| to the coarse pulse-timing circuit 22, in which the pulse is subjected to a different value of time delay also under the control of the barometric device 24. This delayed pulse likewise is amplied and shaped in unit 23 to obtain another delayed pulse having the same amplitude and wave shape as the pulse obtained from unit 2II. The attenuator and wave shaper I2 preferably is adjusted so that the undelayed pulse from generator II has the same amplitude, duration and wave shape as the delayed pulses obtained from units 20 and 23, attenuation in unit I2 being desirable if sufficient ampliiication is not eiected in units 20 and 23. Pulses from the three units I2, 20 and 23 are applied at different times to modulate the wave signal generated in the transmitter I5 and radiated from the antenna I6. This transmittted Wave signal thus has a first prises the first of the two portions'last modulation characteristic comprising two pulse portions, one occurring for any altitude at a constant time to as indicated by line 5I and the other having a time displacement from the time to which varies with altitude as indicated by the time of occurrence line 52. This time displacement or separation of the interrogation pulses is variable under the control of the pressure-responsive device 24 and thus has a value dependent during any given operating period upon the altitude of the interrogator. The altitude range accommodated by the system is divided into a group of predesignated traffic sectors having boundaries indicated by the light horizontal lines in the upper half of the graph of Fig. 3. Thus, speciiic ranges of time separations of the two pulse portions last mentioned, such separation for any given altitude being represented by the horizontal distance from line 5I to a point on the line 52, designate individual ones of the plurality of predesignated altitude sectors.` The time separation may be relatively small or large, as indicated by the distances between lines 5I and 52 at the respective bottom and top portions thereof, so that the first modulation characteristic earlier mentioned is variable over a given range of operating values or time separations to designate by the,

values thereof specic ones of the plurality of traffic sectors available to mobile objects.

Each specic altitude traffic sector may if desired be designated by a single value of time separation, as by using a barometric device which operates in steps when altitude sector boundaries are crossed. However, it may, on the other hand, be more convenient to designate each tralc sector by specic time separations Within a rathersmall range of separations, and this is the type of operation indicated by the slope o1' line 52 within each altitude sector. Of course, it may be convenient in practice to cover the small range of time separations individual to each sector in several small steps rather than as a continuous variation with change in altitude, but the accuracy of designation of the altitude zones or sectors is increased if the delay of the coarse pulsedescribed (and represented by line II) and another or third pulse portion which has a spacing from the iirst pulse portion at time tu varying with altitude within each altitude sector as represented by one of the sloping lines 53 in the upper half of the graph of Fig. 3. These two pulse portions thus occur with a time separation at any given altitude zone within a given sector as represented bythe horizontal distance between line 5I and a point on one of the lines 53 corresponding to the altitude of the interrogator. This time separation is variable under the control of the pressure-responsive device 24 to designate a specic traffic zone located primarly within the traffic sector designated by the ilrst modulation characteristic. As demonstrated by the graph, a specific time separation of the two pulses last mentioned may designate the same (i. e., the fourth) altitude traffic zones in each sector so that the complete designation of' a particular one of the numerous zones depends upon which sector is designated concurrently'with the zone designation. Again, as with the`rst or sector modulation characteristic, it is not necessary that the time separation of the pulses of the zone moduj lation characteristic vary smoothly with changes in altitude. The separation may change in small steps, the altitude diierence required to change from one step to the next determining the altitude sensitivity of the system. The delayed pulse produced at a time varyingwith altitude as indicated by one of the lines 53 is obtained from units I9 and 20 and is referred to as a tine pulse because it aiords a closer or ner designation of the altitude of the transmitter than does the timing of the coarse pulse alone. Thus, the second modulation characteristic is variable over a given range of operating values, specincally the range of time separations between line 5I and the lines 5 3 in Fig. 3, for designatingvby the values thereof specific trailc zones located primarily within a speciiic trafIic sect'or designated by the first modulation characteristic.

To illustrate by a particular example the modulation characteristics of the transmitted wave signal, let it be assumed that the altitude of the interrogatoris H1, indicated in Fig. 3 by a horizontal dot-dash line. The rst or reference pulse transmitted by the interrogator occurs at time to, as mentioned above. The coarse pulse, variable in time relative to the reference pulse in accordance with the altitude sector, occurs at Atime te. Hence, the reference and coarse pulses have a specific time separation to-tc which designates' a speciiic traiic sector occupied by the interrogator. A iine pulse occurs at time t: and has with relation to the reference pulse a yspecific time separation to-t/ which designates the specificl tramo zone occupied by the interrogator.

The modulation-signal pulses thus applied to interrogator transmitter- I5, when the interrogating station is at the altitude H; of Fig. `3, are represented graphically at a in Fig. 4. AThese pulses, obtained from units I2, 20 and 2l are determediate and last pulses in the train of pulses with which the wave signal is modulated. In the described arrangement, the intermediate pulse of each group is the ne pulse and the last pulse is the coarse pulse. The first o r reference pulse of each group thereof is intercepted by antenna 30 at a time tp', indicated at b in Fig. 4, determined by the time of propagation tn-tp between the two stations; Referring now to the lower half of the graph of Fig. 3, the transpondor characteristics are here illustrated in similar fashion to the interrogator characteristics shown in the upper half of the graph; The time of interception of the iirst-or reference pulse at time tp is indicated by line 55 and constitutes for "the receiver a reference time which is the same regardless of the particular relative altitudes of the interrogator and of the transpondor. The pulsesderived by the receiver 3| are applied directly therefrom to anv input circuit of the mixer 32.

The coarse pulse-delay circuit 42 with its associated pulse generator and amplifier 43 and the iine pulse-delay circuit 39 with its associated pulse generator and amplier 40 have respectively longer and shorter time delays for obtaining from the derived pulses respectively long delayed and short delayed pulses of predetermined durations. The two time-delay means 42, `43 and 33, 40 translate with suitable timedelays the pulses applied thereto from the receiver 3l. As illustrated in Fig. 3, the total range of altitudes available-to interrogator and transpondor is the same, and the lower half of the graph is dividedby light horizontal lines into the same predesignated altitude sectors shown in the upper half. The delay introduced by the coarse pulse-delay circuit 42 under the control of barometric device 44 is inis seen to diiler according to the altitude sector occupied by the transpondor much in the same manner and by the same range of values at any altitude sector as does the corresponding delay of the interrogator. The delay introduced by the ne pulse-delay circuit 39, under the control of the device 44, is indicated for each altitude sector by a line 51 and it will be seen that the value of this delay decreases with altitude through a sector but increases with altitude from one sector to another. The reasons for this manner of delay variation will become more fully apparent hereinafter.

If it be assumed that the transpondor station is situated at an altitude Hr, indicated by the dotdash line in Fig. 3, it will be apparent that the reference pulse received at the transpondor at time tp emerges from the coarse pulse-delay circuit 42 and amplifier 43 at a delayed time de after beingsubjected therein to a rather large time delay tp-dc, this being the longer time delay above mentioned. The reference pulse in addition emerges from ine pulse-delay circuit 39 and amplifier 40 at a time d1 after being subjected to a shorter time delay -tp-df. Referring to c in Fig'. 4, there are depicted the three pulses applied from the ne pulse-delay circuit 39 and ampliiier 40 to an input circuit of mixer 32 with the shorter delay equal to tb-d' Likewise, at d in Fig. 4

there are depicted the three pulses applied from the coarse pulse-delay circuit 42 and amplifier 43 to an input circuit of mixer 32 with the longer delay tzr-dc.

l0 y tor, the relationship being established on an initial premise that the transpondor and interrogator are at ythe same altitude. For this condition, the longer time delay tp-dc and the shorter time `delay Atp-d; under control of the transpondor barometric device 44 have such relative values that the longer delay corresponds to and is equal to the longer time separation tu-tc provided at the interrogator and the shorter delay corresponds to and is equal to the time difference tf-tf.- between the spacings of the coarse and iine pulses of the interrogator. It is this manner of variation of the shorter delay which causes the delay to decrease with altitude through each sector but to increase with altitude from sector to sector as indicated by the lines'51 of Fig. 3.

As indicated in Fig. 3, the altitudes Hr and H1 of the transpondor and interrogator stations are assumed the same. -Hence, the particular altitude zone selected at the transpondor station by barometric device 44 is the same zone designated by the pulse separation coding of the interrogating signal under control of the interrogatingstation device 24. The mixer 32, reply transmitter 35 and its antenna 35 at the transpondor station comprise means coupled to the signaldetecting means 30, 3| and to the two time-delay means 39, 40 and 42, 43 for utilizing the pulse signal derived by receiver 3l only when the time delays are such as to place the first pulse of the long delay pulse group represented by d of Fig. 4, the second or intermediate pulse of the short delayed pulse group represented by c of Fig. 4 and the third or last-derived pulse of the group represented by b of Fig. 4 effectively in time coincidence. 'I'he time-delay relationships earlier specied ensure this coincidence of pulses whenever the interrogator and transpondor are in the same altitude zone; Thus, as represented graphically at b, c and d in Fig. 4, the three speciiied pulses are indicated as being in exact time coincidence in conformity with the assumed conditionsv and therefore .are translated by the mixer 32 to actuate the transmitter 35 to transmit a reply wave signal, for example one modulated by a modulation-signal pulse of the type represented by e in Fig. 4. It willbe apparent from the foregoingV description of the system operation that the pulse signals received by the transpondor are utilized only when the received pulses have time separations corresponding to the particular traiiic zone then occupied by the transpondor station. This particular traic zone is at all times automatically The values of the transpondor time delays prodisplay tube, the last-mentioned pulse triggers ay sweep circuit which generates a sweep signal used to deiect the cathode-ray beam of the display tube. From the time te tothe time de, a period equal to the time of propagation to-tp, the last pulse transmitted from antenna I6 propagates to the transpondor, as depicted in a and b of Fig. 4. Since under the assumed conditions this pulse arrives at the transpondor mixer 32 in time coincidence with delay pulses from amplifiers 40 and 48, the mixer is conditioned for translating the pulse to the modulation circuit of reply. transmitter 35. The reply pulse leaves transmitter 85 at time de. as depicted in -e of Fig. 4, and arrives at the responser receiver 28 after the same propagation period to-tp has elapsed. The pulse-modulation components of the received wave signal are derived in the receiver 28 and are applied to an intensity-control electrode of the display tube. A visual indication thus appears on the display tube at a time when, under the control of the sweep signal applied to eiect deflection of its cathode-ray beam, the latter has beendeflected to a position corresponding to the round-trip time of propagation between the stations. Therefore, the position of the visual indication is a measure of the distance between the two stations.

Thus. the coding and decoding arrangements of the system require that the two stations be in the same altitude traffic zone before a reply signal is transmitted by the transpondor to enable the position-display unit 29 to provide a distance indication. Unwanted replies consequently are not received from transpondors in 'other altitude zones. Of course, either `the barometric'device 24 or the barometric device 44 may be temporarily adjusted manually to correspond to the correct setting for any specified altitude zone when it is desired leither to obtain position information from, or to send reply signals to, aircraft in the specified zione. v

Fig. shows more fully the details of certain components employed in an interrogator-responser arrangement of the Fig. ltype, elements of Fig. 5 which correspond to similar elements of Fig. 1 being designated by the same reference numerals.

The antenna system I6 of the present arrangement is shown as including a. reflector 45 which imparts a high directivity to the antenna and is manually movable by a hand crank 46 and gear 41 to orient the directivity as desired. The barometric device 24 includes an. aneroid element 48 to the free end of which is fastened a rack 49. The aneroid element is shown in a position corresponding to a low altitude. Rack 49 is positioned operatively with respect to a pinion gear 50, with which a smaller gear 5I also meshes.

The fine pulse-timing circuit I9 includes an input circuit across which is connected a resistor 52 and the input terminals of a time-delay transmission line 53 terminated at its remote end by a 'resistor 54 having a value of resistance equal to the characteristic impedance of the line 53. The in put resistor 52 preferably also has a value of resistance equal to the characteristic impedance of time-delay line 53. The timedelay line 53 may comprise a succession of line sections having lumped series inductors and lumped shunt condensers, or it may comprise a delay line having distributed series inductance and distributed shunt capacitance. The flne pulse-timing circuit i9 also includes a switch' mechanism 55 having a rotatable contactor 56 coupled to the input circuit of unit through an isolating output resistor 51. Contactor 56 selectively contactsl any of a series of switch segments 58 arranged in a semicircle centered on the axis of rotation of the contactor 56. The segments 58 are insulated from each other by insulating spacers narrower in width than the wiper portion of contactor 56, so that the conthe segments. Additional insulated segments 58. also are provided on the switch mechanism extending in both counterclockwise and clockwise directions from the end segments 58 which are positioned on the upper half of the switch. The segments 58 and 56.. are connected in a clockwise sequence to successive taps on the delay line 53, the first or most counterclockwise sesment 58. being connected to a tap affording a minimum time delay relative `to the input end of the delay line and the last or most clockwise segment 55|. being connected near the remote end of the delay line. n

The fine pulse-timing circuit i9 further includes another delay line and switching arrangement identical in construction to the arrangement just described, corresponding elements of the second arrangement being identified by the same reference numerals primed. 56' on the switch 55' sweeps segments 180 degrees removed from the segments swept by the contactor 56 of switch 55. Both of the contactors 56 and 56' are rotatably driven in synchronism by means of suitable electrically insulated mechanical couplings to the smaller gear 5l, as indicated by dot-dash lines. This se'cond timing circuit is coupled between the units Il and 20 in the same manner as the first timing circuit.

The coarse pulse-timing circuit 22 likewise includes a delay line 60 having an input circuit coupled to the output circuit of the unit H and shunted by a resistor`6l. The remote end of the delay line 66 is terminated by a resistor 52 and is coupled to the input circuit of an amplifier and wave shaper 63. The output circuit of the latter has a resistor 64 connected in shunt thereto and is coupled to the input circuit of another delay line 55 terminated by a resistor 66. Each of the resistors 6|, 62, 64 and '66 has a value of resistance equal to the characteristic impedance of the delay line for which that resistor forms an end termination. The use of the unit 53 between the delay lines 60 and 55 is desirable for the reason that the total delay desired of the delay circuit 22 is so great that undue attenuation and degradation of the translated pulse wave form might otherwise be experienced. A switch mechanism 61 has a contactar 56 arranged to sweep over circularly positioned insulated segments 69. The segments 69 are connected togetherin pairs in a clockwise sequence and these pairs are connected to successive taps on delay line 55 with the most counterclockwise pair .connected to a point near the input terminal of the line. The rotatable contactor 58 is connected mechanically to the pinion gear 55 but is electrically insulated therefrom. The contacter 68 of switch 51 is connected conductively to a sweep control circuit of position-display unit 28 and also is connected through an isolating output resistor 15 to the input circuit of unit 28.

The rotatable reflector 45 of the antenna system is driven by a shaft 15 having a slip ring and co-operating brush device 16 for grounding the reflector. The beam of the cathode-ray tube of unit 28l is deflected radially to provide distance indications as previously described, but is also deflected in an angular direction corresponding at any time to the particular angular position at that time of the reflector 45. To this end, the shaft'15 has a mechanical coupling 11 to the angular-deflection field element of Vthe positiondisplay unit 29.

Neglecting for the time being the functions of tactor is always in contact with at least one of 15 the switch segments 55s and 58" of the respec- The contactar asv-1,271

13 tive switches 55 and 55', the operation oi! the interrogator-responsa' of Fig. 5 is the same as I tactors 56 and 56, of switches 55 and 55' leaves thatof the Fig. 1 arrangement. This operation is represented graphically in amplied form by jhe upper half of the graph oi' Fig. '7. This half of the graph corresponds to the lower two altitude sectors of the upper half of the graph of Fig. 3. The predesignated boundaries between these three sectors are indicated by light horirontal lines |28. Using the same altitude and time designations used in connection with Fig. 3

but with a prime'notation, pulse generator II initiates a series of pulses at a time represented by the vertical line |2| and referred to as time to'. This pulse passes through wave shaper I2 to modulate the wave signal generated in transmitter I5. It also is applied through isolating y resistor I8 to the delay line 53 of the line pulsetiming circuit I9. After the length of time necessary for it to reach the proper tap on delay line 53, the pulse passes through one of the segments through isolatingresistor 2| to the delay line Il of coarse pulse-timing circuit 22 and thenceforward through ampliiier and wave shaper 63 to the delay line 65. The total time delay desired in the circuit 22 is so great that undue attenuation and degradation of the pulse might be expected if the ampliiier and wave shaper 63 were not inserted between the delay lines 60 and 65. After traveling to the proper tap on the delay line 85, the pulse passes through a switching segment 69 of switch 51 to contactor 58 and onward through isolating output resistor 10 to the amplier and wave shaper 23. By properly adjusting amplier and wave shaper 63, and-by properly dividing between delay lines 60 and 65 the total time delay required in the. timing circuit 22, the amplitudes and' shapes of the output pulses from timing circuits I9 and 22 may be made approximately the same.

Referring again to Fig. '7, the pulse translated through coarse pulse-timing circuit 22 and unit 23 arrives at transmitter I5 at a time dependent upon the pressure altitude of the linterrogator station. At the low altitude assumed for this station, the contactor 68 of switch 61 contacts one of the most counterclockwise switching segments 68 producing a time interval from the leading edge of the reference pulse to the leading edge of the coarse pulse oi rather long duration, yet relatively shortcompared with` the still longer intervals available. This time interval is represented in the graph of Fig. 7 by the horizontal distance, for a given interrogator altitude, from the line |2| to a line |22. Assuming the interrogator altitude to be Hi',-represented by the horizontal dot-dash line in Fig. 7, the leading edge .of the coarse pulse actuates transmitter I5 at time te after a time interval to-t.

.Meanwhila the same generated pulse passes through line pulse-timing circuit I9 and ampliiler 20, and its leading edge reaches transmitter I5 at a time represented by one of the solid lines |23 on the graph of Fig. 7. The solid lines |23 have a discontinuity at each of the sector boundaries |28. At the'se altitudes one of the two conthe semicircular upper portion of the respective4 switch while the other of the two contactors enter-s that portion of its switch. Thus, at an altitude represented by a sector line the pulse obtained from timing circuit I9 may be considered to shift from a segment 58 to a segment 58 or vice versa, this shift being from a segment connected to a tap near the end of delay line 53v to a tap nearer the beginning of delay line 53' or vice versa. The function of the additional taps connected to the switch segments 58a and 5855 will be considered hereinbelow. Therefore, it is apparent that'the switch segments 58 and 5 8' alternately are connected in the ne pulse-timing circuit I9 during the passage of the pressuresensitive device 48 through alternate altitude sectors. When the interrogator altitude is H1',

` one of the segments 58 or 58' causes the ne pulse to occur at a time -tf, depicted in Fig. '7, separated from the reference pulse by a time interval lfd-tr'.

As the altitude increases the pressure decreases and. the aneroid device. expands and pushes rack I9 to the left. The resulting rotation of pinion gear 50 causes a clockwise rotation of the coarse pulse contactor 68 of switch 61. The switching segments of switch 61 are shown connected to taps on delay line 65 in pairs, two adjacent segments per tap. As contactor 68 leaves the more clockwise segment of each pair, the time separation between the coarse pulse and the reference pulse increases by one small step. As with the graph of Fig. 3, however, the line |22 in the Fig. 7 graph is conveniently shown as having a constant slope indicating a steady increase in pulse separation with increasing altitude. Nevertheless, as already mentioned, it is often more convenient in practice to eiect stepwise increases of the pulse separation. When this is done the line 22 represents with reference to line I2 average changes of pulse separation over a considerable range of altitudes.v In one specific embodiment of the Fig. 5 Yarrangement the contactor 68 covers fourteen segments, or seven pairs of segments, while the altitude changes through a range equal to four predetermined altitude sectors. This corresponds to four-sevenths of an altitude sector for each stepwise increase in coarse pulse separation. It is assumed that the angular position of pinion gear 55 is directly proportional to altitude in feet.

The mechanical coupling from the gear 5| to the contactors 56 and 5B' of switches 55 and 55 is such that these two contactors also move clockwise with increase in altitude. As with the coarse pulse, the fine pulse separation from the reference pulse also conveniently may increase in a stepwise manner. In such a case the lines |23 of the graph of Fig 7 indicate average changes in the time separation between the fine and reference pulses. In one embodiment of the Fig. 5 arrangement, there are thirty-five small stepwise changes in this ne pulse separation in each altitude sector thus corresponding to a 10:1 ratio of the gears 50 and 5|. Accordingly, the contactors 56 and 56'- make one complete revolution every two altitude sectors and may make many revolutions vcontinuously inthe same sense if the altitude change is great enough.

Whenever a tap on one of the delay lines 53, 53' or 65 is connected through its corresponding switching segment and contactor to the corresponding output isolating resistors 51, 51 or 10, an impedance discontinuity is introduced at the absorbed without reflection in-the terminating resistor. When a contactor is between two segments connected to adjacent taps and in contact with both of these segments, the pulse from the later tap is of such decreased amplitude and comes so soon after the pulse from the rst tap that transmitter I is unable .to respond to the second pulse. Thus, the second tap is not eifective while the switch contactor still rests on theV switching segment connected to the first tap. Likewise, when the altitude is decreasing, the tap corresponding to the smaller delay is eiective as soon as the contactor touches the corresponding segment.

The durations of the interrogating pulses produced by the Fig. 5 arrangement also are indicated in Fig. 7. The reference pulse which starts at a time to', asindicated by the line |2|, ends at a time p1, as indicated Vby a line |24. The

coarse pulse, starting at a time indicated'by line,

|22, ends at a. time p3 indictaed by a line |25. The fine pulse, starting at a time indicated by one of the solid lines |23, ends at a time pz indicated by the corresponding point on one of a series of lines |26. Thus, when the interrogator altitude is H1', the fine pulse starting at time tz' ends at time p2, while `the coarse pulse starting at time te ends at time ps. All these interrogating pulses have durations which are substantially the same and equal to the duration to-pi of the reference pulse.

Fig. 8 is a series of graphs having a 'common time scale and representing the various pulses produced during operation of the interrogator system of Fig. 5, and the transpondor system of Fig. 6 presently to be described, under a specic set of conditions. The graph indicated at a of Fig. 8 illustrates the times of occurrences of the leading and trailing edges of reference, coarse and fine pulses taken from the graph of Fig. 7 for interrogator altitude Hi. Corresponding reference characters designate the times of transmission of the leading and trailing edges of the three pulses just discussed.

Fig. 6 represents suitable circuit arrangements for the several units of the Fig. 2 transpondor, elements in Fig. 6 corresponding to similar elements of Fig. 2 being designated by similar reference numerals. The output circuit of receiver unit 3| is coupled through the isolating resistor 35 to the fine pulse-delay circuit 39, the output circuit of -which is coupled to the pulse generator and amplier 40. The output circuit of unit 45 is connected to the screen electrode of the mixer which is shown as comprised by a pentode' type of tube 32. The output circuit of receiver 3| further is connected through the isolating resistor 4| to the coarse pulse-delay circuit 42, the output circuit of which is coupled to the pulse generator and amplier 43. The output circuit of amplifier 43 is connected to the suppressor electrode of mixer tube 32.` The pressure-responsive device 44 comprises an aneroid barometer which is connected mechanically to a rack |45, meshing with pinion gear |46 which in turn meshes with a smaller gear |41. It is preferable that receiver unit 3| include a conventional amplitude limiter so that the pulses applied to the mixer tube 32 and the two delay circuits have a preselected amplitude regardless of wave-propagating conditions between the interrogator and transpondor stations.

The time-delay means of the delay circuits 35 and 42 are illustrated in forms quite similar to the timing circuits I5 and 22 of Fig. 5, and thev delay circuits include delay lines and switch mechanisms which may be almost identically the same lastho'se in the timing circuits. Specifically, ne

pulse-delay-circuit 33 includesv an input resistor lll across the input end of a delay line |52 having a terminating resistor |53. A switch mechanism |54 has a contactor |55 with two wiper arms ,contacting points 180 degrees apart on the periphery of the switch |54. This switch conveniently may carry a number of segments |56 disposed around its periphery in the same manner as the segments 56 and 55.. of the switch 55 in the Fig. 5 arrangement. All the segments are not used, however. Those in the upper half only of 'the switch are connected to successive delay-line taps in the same way as are the segments 55 ot switch 55, except that the greatest time delay is obtained from the tap connected to the most counterclockwise switch'segment. To ensure that contactor |55 is always in circuit with delay line |52, the f lrst tap on the delay line |52 is connected not only to the most clockwise switching segment |55 in the upper half of switch |54, but also to another segment |55 located just below the most counterclockwisefsegment. The contactor |55 isr connected mechanically but not electrically to the smaller gear |41 so as to rotate clockwise with increasing altitude at the same rate per unit change of altitude as-does the cony tactor. 56 of the switch 55 at the interrogator.

Contactar is connected to an input resistor |51 connected across the input end of another'delay line |55 which is terminated by a resistor |55. Associated with this delay line is a switch mechanism having a rotating contactor |5| and switching segments 52. The most counterclockwise segment |62 is connected` to the input end of delay line |55, while the other segments |52 are connected together in pairs in a clockwise sequence and the pairs are connected to successive taps on the delay line |55. The contactor isconnected to the input circuit of the pulse generator and amplifier 45, and lis rotatable in a clockwise sense with increasing altitude by virtue of a mechanical connection to the pinion gear |45.

`The pulse generator and ampliiier 45 has a coupling condenser |63 which couples the contactor |5| of the switch mechanism |65 to the control electrode of an electron tube |64. The input circuit of tube |64 includes an input resistor.`

|55 connected to its control electrode, and a source of biasing voltage |56 connected betweenr -the resistor |55 and ground. The resistor |55 is shunted by a diode electron tube the cathode of ing of transformer |55 to a resistor |1| coupled across the input end of a delay line. |12. 'I'he end of resistor remote from tube |63 is v grounded through a source of biasing voltage |18. Transformer |68 has an output winding -Simllar to those of unit 48.

one terminal of kwhich is coupled to a control electrode of a triode electron tube |14 and the other terminal of which is connected to theI negative terminal of a source of biasing voltage |15. The anode electrode of tube |14 is connected to a source of space current |16, while the cathode is connected through a cathode 'resistor arman |11 to a tap on the source of biasing voltage |15.

' |85 of another delay line |86, terminated in turn by a resistor |81. Associated with delay line |86 is a switching mechanism |88 having a rotatable contacter |89 mechanically coupled to pinion gear |46 for clockwise rotation with increasing altitude. Switch |88I has peripherally arranged switching segments |88, the most counterclockwise of which is connected to a tap on the input end of delay line |86. The other segments |98 are connected in pairs in a clockwise sequence and the pairs are connected to successive taps on delay line |86. Thus, both of the 'switching mechanisms |68 and |88 are similarly connected .to their respective delay lines |58 and |86 and may have contactors similarly mounted on the same shaft. Both mechanisms resemble in construction and operation the switch 61 at the interrogator. The delay lines 65 and |86 may be identically constructed from their rst taps to their remote ends.

Contactor |89 of switch |88 is' connected to the input circuit of the pulse generator and amplier 43. Unit 43 ismade up of components Thus its input circuit, following a coupling condenser |93, includes the control electrode of a triode tube |94 connected to ground through a resistor |95 and biasing battery |96. Resistor |95 is Shunted by a diode |81. Another trlode |98 and the tube |94 have a common anode-cathode circuit,` and their anodes are connected through a winding of a transformer |99 to a source of space current 288. Another transformer winding is connected to the control .electrode of tube |98 and. through a resistor 28| in shunt with the input end of a delay line 282, to a biasing battery 283 and ground. An output winding of transformer |99 is'coupled to a control electrode of a triode tube 284, and to a source of biasing potential 286. Tube 284 has'a source of space current 286 and a cathode resistor 281 grounded through a tap on battery 285. The cathode of tube 284 also is connected to the suppressor electrode of mixer tube 32. The delay lines |12 and 282 of units 48 and 43, respectively, are open-circuited at their remote ends, and usually introduce different amounts of delay.

The operation of the transponder station illustrated in Fig. 6Lwil1 be considered with reference to the graphs of Figs. 7 and 8. The lower half of the graph of Fig. 7 represents the pulse- 18 light lines |28. A line 2|5 represents the time of interception of the leading edge of the reference pulse as received by the transpondor from the interrogator station. This time is designated tp', giving a time of propagation between the stations equal to tlf-tp. The intercepted pulses are illustrated at b in Fig. 8, starting as the time tp'. As intercepted, their durations are the same as when transmitted but their amplitudes are determined by the limiting level imposed in receiver unit 3|. In the absence of these pulses the screen electrode of mixer tube 32 is at ground potential, and the tube can conduct no appreciable space current regardless of the voltages present on the control and suppressor electrodes.

'I'he intercepted. pulses also pass through the ne pulse-delay circuit 39 and appear with positive polarity at the input circuit of unit 48, as

illustrated in c of Fig. 8. Although these pulses may be subject to some attenuation and deterioration of their'wave form` inthe delay circuits |52 and |58, they still have suilicient amplitudes to overcome the bias of battery |66 and render tube |64 conductive. The resulting space current is supplied to tube |64 from battery |18 through the anode winding of transformer |69. As tube |64 starts to conduct, the current through the anode winding induces a voltage impulse on the winding which is connected to4 the control electrode of tube |68. The latter winding is connected to this control electrode and to delay line |12 in such a sense that the voltage impulse is applied to the control electrode with positive polarity to cause control-electrode current to flow. The control-electrode current produces across the resistor |1| a. potential pulse which is applied to the delay line |12 with negative polarity. As a result, tube |68 also commences to draw space current through the anode winding of transformer |69. This space current continues to increase at a rate suiiicient to cause a continuing positive voltage to be induced in the winding connected to the control electrode of tube |68 even after the pulse applied to tube |64 ceases. When the applied pulse ceases, coupling condenser |63 may be left with a negative charge but this charge is quickly dissipated through diode |61 to prepare circuit 48 for the next positive pulse. Meanwhile, the leading edge of the negative voltage impulse applied to delay line |12 travels down'this line to the open end thereof, where it is reiiected without change in polarity, and travels back to the input end of the line. At that time the, reflected voltage impulse is applied with negative polarity to the control electrode of tube |68 and causes the latter to stop conducting. Thus, a voltage is induced in the output winding of transformer |69 for a period of time commencing with the application of a delayed pulse to the control electrode of tube |64 and ending when the reflected pulse returns to the input end of delay line |12. This voltage is applied during this period with positive polarity to the control electrode of tube |14 to overcome the bias of'battery |15. Hence a pulse of the same duration is developed with positive polarity across the cathode load resistor |11 and is applied tothe control electrode of mixer tube 32 to overcome the bias supplied thereto from the tap of battery |15.

Each of the delayed pulses of c in Fig.V 8 is applied to the input circuit of the pulse generator and amplier 48 and the leading edge of each triggers the blocking oscillator 68 into a conductive condition to cause the generation of the longer pulses 'depicted in d of Fig. 8, which I circuit 42. In this circuit they are delayed in delay line |82 and thenapplied to amplifier and wave shaper |84 to compensate for attenuation and degradation ofthe wave shape occurring in delay line |82. The amplified pulses pass along delay line |86 to the tap selected by switch |00 and then pass from contactar |39 to the input circuit of pulse generator and amplier 43. The latter operates in a manner exactly analogous to the operation of the pulse generator andampliiier 40. However, the output pulses from unit V43 are of shorter duration than those from unit 40 due to the use ot a delay line 202 in the blocking oscillator of unit 43 having a relatively short time delay. The resulting pulses are applied from cathode load resistor 201 of tube 204 t the suppressor electrode of mixer 32 to overcome. the negative bias supplied to this electrodefrom the tap on battery 205. It at the same time a signal is applied from circuit 40 to overcome the bias applied to the control electrode and a pulse from the output circuitof the receiver unit 3| simultaneously provides a suitable voltage at the screen electrode, tube 32 is conditioned to conduct space current. The delayed pulses-applied from coarse pulse-delay circuit 42 to pulse generator and ampliiier 43 are illustrated in the graph e of Fig. 8, and the corresponding pulses applied from unit 43 to mixer tube 32are illustrated in f of Fig. 8.

' Referring again to the lower half of .the graph of Fig. 7, line 2|6 represents the start of the reference pulse applied to the mixer tube after passage through coarse pulse-delay circuit 42 and triggering of the pulse generator and amplifier 43. Thus, the time delay from the start of the intercepted reference pulse to the start of the delayed reference pulse applied to the suppressor electrodeA of the mixer tube is represented by the horizontal distance, for a given altitude of the transpondor, between the lines 2|5 and 2|6. Assuming that the replier has a pressure altitude Hr near the lower boundary of the second lowest altitude sector, this delayed pulse starts at'a time dc after a delay tlf-de from the start of the intercepted reference pulse. The time de also is noted in f of Fig. 8, where it is seen to be the time of starting of the reference pulse after vdelay in coarse pulse-delay circuit 42, as shown in e of Fig. 8, plus a very small additional delay required to trigger into operation the blocking oscillator circuit of unit 43. All the intercepted pulses are delayed in unit 42 and cause correspending pulses of extended duration to be genator and amplifier 40 to mixer tube 32 starting at a time represented by one of the lines 2|1 in Fig. 1. This delayed reference pulse is followed by other pulses corresponding to the other intercepted pulses and equally delayed and extended in duration. When the altitude of the replier is Hr', the delayed reference pulse applied to the control electrode or mixer tube 32 from unit 40 the receiver unit 3| to the coarse pulse-delay starts' at the time di'. as :unina-maa in '1.

reference pulse after suffering the tlne delay to a starting time dr' ends at a time p5 with a-pulse duration df-ps determined by delay line |12. Likewise, the reference pulse after suffering the coarse delay to a starting time de' ends at a time pa with a. pulse duration diy-ps determined by the delaykline 202. The` times p6 and pa also are indicated in connection with the representations of the delayed reference pulses in graphs d and f, respectively. of Fig. 8.

As previously explained in connection with the systems of Figs. 1 and 2, the transpondor mixer. 32 utilizes the pulses of the intercepted composite signal provided there are applied to the three input circuits of the mixer three pulses eiectively in time coincidence. The intercepted pulses of the composite signal are applied directly from the receiver unit 3| to the screen electrode of tube 32. After a longer or coarse delay, the intercepted pulses are applied to the suppressor elec-` trode of the mixer tube. This delays the nrst or reference pulse by an amount suilicient to make it coincide with the last pulse applied to the screen electrode, provided the interceptedy pulses are coded for the altitude sector selected at the transpondor. Hence, theftranspondor has a long time delay which providesia rst selective or decoding characteristic determining the particular one of the predesignated series of broad altitude sectors. After a shorter or nne delay. the intercepted pulses are applied to the control electrode of tube 32. This delays the intermediate or ne pulse by an amount suicient to make it coincide with the last pulse applied to the screen electrode, provided the intercepted pulses arer coded for the altitude zone selected at the transpondor. Hence, the transpondor also has a short- Aer time delayv which provides a second selective characteristic fixing the altitude zonal position of the transpondor within the particular altitude sector selected at the transpondor.- Since in this case the `ilne pulse is delayed into coincidence with the coarse pulse, the coarse pulse timing also must be taken into account in setting the ilne delay; this is accomplished in the Fig. 6 arrangementXby adding an increment of delay from delay line |58 to the delay of delay line |52. The addition of the delay of delay line |58 causes the zone decoding characteristic limits to vary from sector to sector.

The transpondor itself also provides utiliza.-l tion of the received composite signal whenever the interrogator and transpondor stations are within a predetermined altitude tolerance even though one of them is slightly within one of the sectors and the other is slightly'within an adjacent sector. The two delay lines |12 and 202, in establishing the pulse duration of the delayed pulses applied from the units 40 and 43 to the mixer tube 32, determine this separation-tolerance.

The interrogator preferably provides an additional amount of altitude separation tolerance at the boundary of two adjacent zone sectors.

To this end, an additional intermediate or nne- .rst sector.

2l pulse is included in the composite interrogator signal whenever the principal iine pulse represented in Fig. '1 by a solid line |23 occurs at a time approaching one of the sector limits. The starting time of the additionalne pulse is represented in Fig. rI by a portion oi one o! a group of dotted lines 223, which are extensions of the solid lines |23 in -both directions. The lowest line |23 is not extended in the direction representing lower altitudes because the lower` end of that line corresponds to the lowest altitude accommodated by the system. The extents of the dotted extensions 223 are' determined by the predetermined altitude tolerance of the transponder system, and hence are related to the duration of the short delayed pulses obtained at the transpondor. Now the last-mentioned duration represents` double the desired altitude tolerance to take into account transpondor altitudes both greater and less than the interrogator altitude plus a small increment or duration to assure coincidence for a period long enough to .actuate the mixer unit. Accordingly, each dotted extension 223 extends far enough to represent a timing change equal to slightly lessi than onehalf of the duration of the short delayed pulses of the transpondor. To demonstrate the' function of theauxiliary ne pulse, reference again is had to Fig. '7. The altitude sector appropriate to the interrogator altitude Hi is the lowest or For interrogator altitude H1" the principal ne pulse starts at time tf', as determined from the solid line |23 for the first sector.

The starting time of .the auxiliary ne pulse is ta, as determined from the dotted line 223 which is an extension into the iirst sector of the line |23 for the second sector. The second sector is the particular sector-occupied bythe replier at altitude Hr'. Thus the last-mentioned dotted line 223 represents variations of the auxiliary `ne pulse beyond the predetermined lower limit of the last-mentioned line |23. The auxiliary ne pulse conveniently has the same duration as that of the other interrogator pulses. Accordingly, in Fig. 7 the auxiliary pulse which starts at a time te. as indicated by a dotted line 223, ends at a time p4 as indicated by a dotted line 226. This and the other dotted lines 226 are extensions of the solid lines |26. The auxiliary fine pulse starting at time t. and ending at time p4 appears in broken lines in graph a of Fig. 8 along with the reference pulse. principal fine pulse and coarse pulse. If all four pulses, including the rst or reference pulse. the two intermediate or iine pulses and the last or coarse pulse, are intercepted at the transpondor, they all give rise to the corresponding short delayed and long delayed pulses as depicted by both the broken-line andsolid-line curves of graphs b-f,

and 55' adjacent the segments 58 and 58' for the principal fine pulses. It is evident from an inspection of the delay-line taps, switch segments and switch contactors 56 and 56' that the timing of the principal and auxiliary fine pulses corresponds in general to the timing depicted by the lines |23 and dotted extensions 223 in the graph of Fig. 7. `It will be apparent that the arrangement of Fig. 5 provides vfine stepwise timing intervals for the auxiliaryilne pulse near each end of an altitude sector.

Referring again to Fig. 8, it is of interest to examine the intercepted pulses, short delayed pulses and long delayed pulses of b, d and l, respectively, to ascertain the manner in which pulses are applied simultaneously to the three input electrodes of mixer tube 32 so as to actuate the reply transmitter 35. The last or coarse pulse as intercepted starts at the time t5 and ends at the time tr', as appears from b in Fig. 8. After subjection to the long or coarse delay, the iirst or reference pulse produces a pulse depicted in f of Fig. 8 between the times do' and ps. a duration encompassing th'e duration of the last intercepted pulse. Graph d of Fig. 8 indicates, however, that the principal fine pulse (the second pulse from the right in d of Fig. 8) after subjection to the short or i'lne delay'occurs substantially later than the time tr' so that the transpondor would not respond to this pulse spacing to transmit a reply wave-signal pulse since the pulse spacing is that occurring when the interrogator and transpondor are in 'diierent altitude sectors. The presence of an auxiliary line pulse in the interrogator pulse coding indicates, however, that the interrogator is near the boundary of two adjacent sectors. If the interrogator is in one and the transpondor is in the otherof these adjacent sectors, the auxiliary ilne pulse after delay in the fine pulse-delay circuit starts some time before the time t5 and ends at the time tr', so that it encompasses the duration of the last intercepted pulse. Consequently, signals are applied, to all three, input electrodes or the mixer tub`e 32 coincidentally starting at time t5 and ending at tr'. Assuming the duration of coincidence necessary to actuate reply transmitter 35 is ts-tr', the reply signal commences at time tr'. Such a reply signal isv depicted in g of Fig. 8. It may be noted that even a slightly greater altitude separation of the two stations would prevent the transpondor from operating .since with a slightly lower interrogator altitude the short delayed auxiliary ne puise, which as depicted in dof Fig. 8 lasts just long enough to permit actuation of transmitter 35, would start even sooner and would end too soon for sufcient coincidence with the other two pulses in mixer 32. Thus the duration of the short delayed pulses determines the altitude tolerance of the system'by determining the extent of the particular altitude zone selectable at the transpondor.

By obtaining coincidence in mixer 32, the transpondor has been conditioned for transmitting a reply Wave signal in response to a suitable received composite signal. The reply wave signal may be given identifying characteristics by suitable means (not shown) provided in the reply transmitter 35; for example, the transmitted reply wave signal may be modulated in accordance with the vMorse character N as depicted in g of Fig. 8 to identify the transpondor.

Fig. 9 represents the type of displaywhich may be produced by the position-delay unit 29 of Fig. 1. It will be noted that the 'display includes two pulses corresponding to the Morse character N which identifies the transpondor station of Fig. 2. The distance from the left hand or starting point of the sweep to the leading edge of the iirst or broad pulse denotes the distance between the interrogato'r-responser station of Fig. l and the transpondor station of Fig. 2. The screen oi.' the display unit 23 preferably carries a suitable scale asvaavi 23 l to indicate distance. Thus the display unit 29 is effective to indicate both the position and the identity of the transponder station.

It will be apparent from the foregoing description of the invention that a communication system embodying the invention is able, by means of easily controlled pulse coding, accurately and consistently to communicate selectively with any of a very large number of communication channels. The communication system of the invention has the additional advantage that it enables a substantial simplification and reduction of cost of the equipment necessary to eifect such selective communication as compared with other systems heretofore proposed for this purpose. Additionally, the communication system of the invention is characterized by such high accuracy and reliability of operation as to render it particularly suitable for aircraft navigation and traffic control applications.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object comprising. means at one of said stations for transmittingwavesignal energy having a first modulation characteristic including two pulse wave form portions with a variable time separation for designating by the values of the time separations thereof individual ones -of a plurality of trafilc sectors available to mobile objects and having a second modulation characteristic including one of said two pulse wave form portions and including another pulse wave form portion with a variable time separation designating by the values of the time separations thereof individual ones of a plurality of traflc zones located primarily within eachof the traffic sectors individually designated by said first characteristic, means at the other of said stations for intercepting said wave-signal energy and for deriving therefrom signal energy including three pulse wave form portions having said two time separations, and means at said other station for utilizing said derived signal energy only when the time separations of the pulse wave form portions thereof correspond to a particular traffic sector and zone selectable at said other station.

2. A system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object comprising, means l at one of said stationsfor transmitting wavesignal energy modulated by a signal of pulse wave form having a. separation between a first pair of pulses thereof of controllable value designating individual ones of a plurality of traffic sectors available to mobile objects and having a separation between another pair of pulses lthereof designating by individual ones of a plurality of traifc zones located primarily within each of the trafiic sectors individually designated by said rst pair of signal pulses, means at the other of said stations for intercepting said wavesignal energy and for deriving therefrom irst, intermediate, and last pulses having said two time separations between pairs of pulses thereof,

24- two time-delay means having longer and shorter time delays -respectively for obtaining from said derived pulses respectively long delayed and short delayed pulses, and means coupled tosaid intercepting means and to said two time-delay means for-utilizing said derived pulses only when the first long delayed pulse, the intermediate short delayed pulse, and the last derived pulse are effectively in time coincidence, said longer and' shorter time delays being selected so that said longer delay corresponds to the longer of said two time separations and said shorter delay corresponds to the time difference between said two time separations when said time separations designate va particular tramo sector and zone selectable at said other station.

3. In a system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object, a wave-signal receiver comprising, means for intercepting wavesignal energy having a first modulation charaeteristic including two pulse wave form portions with a variable time separation for designating by the values of the time separations thereof individual ones of a plurality of tramo lsectors available to mobile objects and having designated by said first characteristic. means for ifi deriving from said intercepted wave-signal energy including three pulse wave form portions having said two time separations, and means for utilizing said derived signal energy only when said time separations of the pulse wave (form portions thereof correspond to a particular traillc sector and zone selectable at said receiver.

4. In a system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object and which includes means for transmitting wave-signal energy modulated by a signal of pulse wave form having a separation between a first pair. of pulses thereof of controllable value designating individual ones of a plurality of traflic sectors available to mobile objects and having a separation between'another pair of pulses thereof designatingA individual ones of a plurality of traillc zones located primarily within each of the traflc sectors individually designated by said first pair of said signal pulses, a wave signal receiver comprising: means for intercepting said wave-signal energy; demoduiation means for deriving from said intercepted wave-signal energy first, intermediate, and last pulses having said two time separations; two time-delay means having longer and shorter time delays respectively for obtaining from said derived pulses respectively long delayed and short delayed pulses; and means coupled to said demodulation means and tol said two time-delay means for utilizing said derived pulses .only when the first long delayed pulse, the intermediate short delayed pulse, and the last derived pulse are effectively in time coincidence, said longer and shorter time delays ybeing selected so that said longer delay corresponds'to the longer of said two time separations and said shorter delay corresponds to theltime difference between said two timeseparations when said time separations designate a particular traiilc sector and zone selectable at said receiver. v

5. A system for communication between two spaced wave-signal stations at least one of which is carried by a mobile object comprising: means at one of said stations for transmitting wave signal energy having a first characteristic variable over a given range of values for designating by the values thereof individual ones of a plurality of trame sectors available to mobile objects and having a second pair of characteristics variable over two ranges of values for designating by one of said two ranges of values individual ones of a plurality of traillc zones located primarily within each of the trafiic sectors individually designated by said first characteristic and for designating by the other of said two ranges of values a region near the boundary of two adjoining ones of said traiilc zones; means at the other of said stations for intercepting said wave-signal energy and for deriving therefrom signals having a characteristic of which three values represent individual ones of the values of said three characteristics; and means atfsaid other station for utilizing said last-mentioned signals only when' two values of,

said characteristic thereof correspond to a particular traillc sector and zone selectable at said other station or when the third of said values corresponds to a region near the boundary of a trailic zne adjoining said particular trame zone selectable at said other station.

6. A system for communication between two` 26 sectors available to mobile objects, said wavesignal energy having a second modulation characteristic including one of said two pulse wave form portions and including another pulse wave form portion with a variable time separation designating by the values of the time separations thereof individual ones of a plurality of trailic zones located primarily within each -of the traffic'sectors individually designated by said first characteristic, said wave-signal energy also having a third modulation characteristic including said one of said two pulse wave form -portions with a variable time separation designating by the values of the time separations thereof individual regions near the boundaries of/two adjoining ones of said trailc zones; mea/ns at the other of said stations for intercepting said wave-signal energy and for deriving therefrom signal energy including four pulse wave form portions having said three time separations; and means at said other station for utilizing said derived signal energy only when the time separations of the vpulse wave form portions thereof correspond to a particular tratlic'sector and zone selectable at said other station or to a region near the boundary of a traffic zone adjoining said selectable trailic zone.

ARTHUR V. LOUGHREN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number y Herbst Dec. 6, 1949 

